The present invention relates to a method and a device for operating a cryogenic tunnel, which tunnel is of the type through which products to be chilled or deep-frozen pass and is equipped with means for injecting a cryogenic fluid as well as means for extracting the cold gases resulting from the vaporization of the fluid in the tunnel at a variable rate.
A cryogenic tunnel is an open system through which products pass, which are intended to be chilled or deep-frozen by injecting generally liquid nitrogen or some other cryogenic fluid which needs to be removed from the system in the form of a gas after vaporization.
The tunnel has an opening through which the products can enter and an opening through which the products can leave.
The cryogenic liquid enters the tunnel through one or more pipes.
One or more additional openings are generally dedicated to extracting the cold gases resulting from the vaporization of the fluid in the tunnel, which therefore entails pumping out the gases containing a large proportion of nitrogen and discharging them to the external surroundings.
In ideal operation, the gas flows should be balanced as follows:                Extraction rate=flow rate of nitrogen gas generated by the liquid nitrogen injection.        Product exit side: air intake rate zero, and gas release rate also zero.        Product entry side: ditto i.e. air intake rate zero and gas release rate both zero.        
It is virtually impossible to obtain such ideal operation in practice and, in particular, it is very difficult to control the following two aspects in a consistent way:                Matching the extraction rate to the volume of nitrogen gas generated: the quantity of nitrogen injected into the tunnel is variable in practice, and it may therefore be difficult to make the extraction keep pace with the requirements.        Balancing the gases between the entry and the exit of the tunnel: a tunnel may have a slightly negative pressure on the product exit side and a slightly positive pressure on the product entry side if the extraction rate is matched correctly, even though the situation may become reversed a moment later.        
Various approaches have therefore been proposed in order to provide solutions to the problems listed above.
Most frequently, “over-extraction” is performed in order to prevent releases of gas (and therefore leaks of nitrogen into the production premises).
This typically involves extraction at a fixed rate, which is calculated with a large safety margin relative to the maximum requirements of the tunnel, with suction hoods being located at the entry and exit of the tunnel.
The following characteristics are observed in such a case:                the extraction rate is much more than the flow rate of nitrogen gas generated by the liquid nitrogen injection.        Product exit side: the air intake rate is much more than 0, while the gas release rate is almost zero.        Product entry side: ditto i.e. an air intake rate much more than 0, while the gas release rate is almost zero.        
It will therefore be understood that the advantage of this technical solution is that the risk of anoxia (cumulative nitrogen leaks in the production premises leading to a reduced level of oxygen in the room) is low when the tunnel is started up, but its drawback is associated with the large intakes of air which cause moisture to enter the tunnel. On the inside, the equipment therefore ices up rapidly and loses its efficiency. This intake of air also leads to an over-consumption of nitrogen.
It should be noted that these intakes of air also cause moisture to enter the extraction lines, and therefore the creation of ice in them. After several hours of operation, this ice may obstruct the extraction lines and lead to nitrogen leaks from the tunnel due to lack of extraction (whence a risk of anoxia).
Another solution encountered quite frequently in the industry, in order to limit the intakes of air and releases of gas, is one according to which the extraction is only slightly more than required (“slight over-extraction”). This is often the best compromise which can be found in the state of the art.
According to this solution, extraction is performed at a fixed rate which is calculated to be just above the maximum requirements of the tunnel, or alternatively variable-rate extraction indexed to the degree of opening of cock letting liquid nitrogen into the tunnel.
The following characteristics are observed in such a case:                the extraction rate is more than the flow rate of nitrogen gas generated by the liquid nitrogen injection.        product exit side: the air intake rate is slightly positive, with greater or lesser variations according to the operating phases of the tunnel, while the gas release rate is slightly negative on average, here again with greater or lesser variations according to the operating phases of the tunnel.        product entry side: here again the air intake rate is slightly positive on average, while the gas release rate is slightly negative on average.        
It can therefore be seen that the balance between the exit and the entry of the tunnel may vary over time, and that an observable situation in which gases are released from the entry of the tunnel and air is taken in at the exit of the tunnel may change to a situation in which air is taken in at the entry of the tunnel and gases are released from the exit of the tunnel.
It will therefore be understood that the advantage of this “slight over-extraction” solution is that the risk of anoxia is quite low when the tunnel is started up, while its major drawback, just as in the case of over-extraction, is associated with the fact that the intake of air causes icing of the equipment and of the extraction lines, and an over-consumption of nitrogen. The air intake rate, however, is low and the technical drawbacks listed above are then more or less limited depending on the case.
A last approach may also be mentioned, although it is almost never employed in practice, which involves using reduced pumping in order to limit the intakes of air (“under-extraction”).
The following characteristics are observed in such a case:                an extraction rate less than the flow rate of nitrogen gas generated by the liquid nitrogen injection.        product exit side: an almost zero air intake rate, while the gas release rate is positive.        product entry side: again, an almost zero air intake rate with a positive gas release rate.        
The advantage of the situation is indeed that no air is taken in at the entry and exit of the tunnel. No ice is therefore deposited in the equipment and in the extraction lines, and there is no over-consumption of nitrogen due to possible intakes of hot air.
But it is quite clearly dangerous to operate a tunnel under these conditions. The leaks of nitrogen to the outside of the tunnel entail a risk of anoxia and therefore a situation which is dangerous for the personnel working nearby.
The above discussion therefore demonstrates the genuine need to be able to provide a solution that offers a better compromise for this industry, making it possible to work closer to the ideal equilibrium. To that end:
the extraction rate should be matched to the volume of nitrogen gas which is generated. Since the quantity of nitrogen injected into the tunnel is variable, the extraction rate should also keep pace with the requirements as accurately as possible while allowing for the possible lags between the injection of liquid nitrogen and the moment when it vaporizes.                concerning the balance of the gases between the entry and exit of the tunnel: the system should make it possible to guide the gases in order to prevent them being released from either the entry or the exit of the tunnel.        all these checks are preferably automatic without any human intervention other than fixing the initial settings.        
With such balancing of the gases in the tunnel and an extraction which is fully matched to the requirements, the tunnel would thus no longer take air in (either at the entry or at the exit) and could therefore operate for a longer time without de-icing and without losing its efficiency. The extraction lines would no longer be obstructed, and the leaks of nitrogen would at the very least be significantly reduced or even eliminated. This would overcome the risk of anoxia.
The approach of Document U.S. Pat. No. 5,878,582 may also be mentioned, which attempts to control a cryogenic chamber by comparing a temperature value at the external entry of the tunnel with a setpoint, and by feedback control of the extraction means of the chamber according to the result of this comparison.
The Applicant has been able to show that although this technical approach offers certain improvements over the prior-art approaches mentioned above, it is still unsatisfactory quite simply because it does not take account of the ambient temperature in the premises where the cryogenic chamber is operating.
Specifically, the setpoint temperature should be close to the ambient temperature in order to obtain good results according to this document, while always remaining lower than it. This is because if the setpoint becomes higher than the ambient temperature (since the ambient temperature has fallen), then the system becomes inoperable because the extraction will accelerate endlessly but without ever being able to reach this setpoint temperature. It will be impossible to increase the measured temperature above the temperature of the ambient air. In short, the system can be controlled easily according to this technique if the ambient temperature in the premises is relatively stable (plus or minus one degree), but when the temperature of the premises varies (which is often the case in food production premises) then this control technique may become inefficient or occasionally inoperable (setpoint temperature becoming higher than the ambient temperature).
In one aspect of the present invention a method for operating a cryogenic tunnel through which products to be chilled or deep-frozen pass is provided. This tunnel is equipped with means for injecting a cryogenic fluid as well as means for extracting, at a variable rate, some of the cold gases resulting from the vaporization of the fluid in the tunnel. The method includes obtaining a gas temperature, wherein this gas temperature comprises a value selected from the group consisting of the temperature of the gases in proximity to the entry to the tunnel, and the temperature of the gases in proximity to the exit to the tunnel, wherein this gas temperature is obtained from at least one gas temperature probe which is provided outside the tunnel, at a location selected from the group consisting of proximity to the tunnel entrance, and proximity to the tunnel exit. The method also includes obtaining an ambient temperature, wherein this ambient temperature is obtained from at least one ambient temperature probe which is provided outside the tunnel. The method also includes determining a first delta, wherein this first delta is the difference between the ambient temperature and the gas temperature. The method also includes comparing the value of the first delta with a first setpoint value. And the method includes controlling the extraction rate of the extraction means by feedback as a function of the result of the comparison in step d), in order to restore the value of the first delta to the setpoint value if necessary.
In this context, the invention relates to a method for operating a cryogenic tunnel through which products to be chilled or deep-frozen pass, which tunnel is equipped with means for injecting a cryogenic fluid as well as means for extracting, at a variable rate, some or all of the cold gases resulting from the vaporization of said fluid in the tunnel, characterized in that:
a) at least one temperature probe is provided outside the tunnel, in proximity to its entry and/or its exit, which is capable of providing a value Tentry/exit of the temperature of the gases at the point where it is located;
b) at least one temperature probe is provided outside the tunnel, which is capable of providing a value Tamb of the ambient temperature of the premises where the tunnel is operating;
c) the difference Tamb-entry/exit between said ambient temperature Tamb and said temperature Tentry/exit is determined, or alternatively the difference between the average of the ambient temperatures which are provided by said ambient temperature probes and the average of said temperatures Tentry/exit which are provided by said entry/exit temperature probes;
d) the value of the temperature difference provided by step c) is compared with a predetermined setpoint value T0amb-entry/exit;
e) the extraction rate of said extraction means is controlled by feedback as a function of the result of the comparison in step d), in order to restore the value of said temperature difference to said setpoint value T0amb-entry/exit if necessary.
The Applicant has therefore demonstrated the fundamental importance of taking into account the ambient temperature of the premises where the tunnel is operating, in order to obtain high-quality operation. It can be seen that the ambient temperature probe should preferably be arranged at a position where the temperature is not influenced by the tunnel or by any other machine or ventilation system which may be present in the premises in question.
The operating method according to the invention may furthermore adopt one or more of the following technical features:                regulation of the PID type is used in order to carry out said feedback in step e).        one or more gas equilibration valves are provided inside the tunnel, which is/are capable of directing the cold gases to the entry or the exit of the tunnel and can be actuated automatically from outside the tunnel.        in the case when said valves are present:        
i) at least one temperature probe is provided outside the tunnel, in proximity to its exit, which is capable of providing a value Texit of the temperature of the gases at the point where it is located, and at least one temperature probe is provided outside the tunnel, in proximity to its entry, which is capable of providing a value Tentry of the temperature of the gases at the point where it is located;
j) the difference Texit-entry between said temperature Texit and said temperature Tentry is determined, or the difference between the average of the temperatures Texit which are provided by said exit temperature probes and the average of said temperatures Tentry which are provided by said entry temperature probes;
k) the value of the temperature difference provided by step j) is compared with a predetermined setpoint value T0exit-entry;
l) the orientation of some or all of said equilibration valves is controlled by feedback as a function of the result of the comparison in step k), in order to direct some or all of the cold gases contained in the tunnel so as to restore the value of said temperature difference to said setpoint value T0exit-entry if necessary.                regulation of the PID type is used in order to carry out said feedback in step l).        said extraction means on which the feedback is carried out comprise a single extraction line located inside the tunnel, substantially above the region where the products enter.        
The invention also relates to a device for operating a cryogenic tunnel through which products to be chilled or deep-frozen pass, which tunnel is equipped with means for injecting a cryogenic fluid as well as means for extracting, at a variable rate, some or all of the cold gases resulting from the vaporization of said fluid in the tunnel, comprising:
a) at least one temperature probe located outside the tunnel, in proximity to its entry and/or its exit, which is capable of providing a value Tentry/exit of the temperature of the gases at the point where it is located;
b) at least one temperature probe located outside the tunnel, which is capable of providing a value Tamb of the ambient temperature of the premises where the tunnel is operating;
c) a data acquisition and processing unit capable of determining the difference Tamb-entry/exit between said ambient temperature Tamb and said temperature Tentry/exit, or alternatively the difference between the average of the ambient temperatures which are provided by said ambient temperature probes and the average of said temperatures Tentry/exit which are provided by said entry/exit temperature probes, of comparing the value of the temperature difference provided by the previous step with a predetermined setpoint value T0amb-entry/exit, and of optionally controlling the extraction rate of said extraction means by feedback as a function of the result of the previous comparison, in order to restore the value of said temperature difference to said setpoint value Tamb-entry/exit if necessary.
The operating device according to the invention may furthermore adopt one or more of the following technical features:                the data acquisition and processing unit uses a regulator of the PID type in order to carry out said feedback.        the device comprises one or more gas equilibration valves inside the tunnel, which is/are capable of directing the cold gases to the entry or the exit of the tunnel and can be actuated automatically from outside the tunnel.        in the case when said valves are present, the device also comprises:        
i) at least one temperature probe located outside the tunnel, in proximity to its exit, which is capable of providing a value Texit of the temperature of the gases at the point where it is located, and at least one temperature probe located outside the tunnel, in proximity to its entry, which is capable of providing a value Tentry of the temperature of the gases at the point where it is located;
j) a data acquisition and processing unit capable of determining the difference Texit-entry between said temperature Texit and said temperature Tentry, or the difference between the average of the temperatures Texit which are provided by said exit temperature probes and the average of said temperatures Tentry which are provided by said entry temperature probes, of comparing the value of the temperature difference provided by the previous step with a predetermined setpoint value T0exit-entry, and of optionally controlling the orientation of some or all of said equilibration valves by feedback as a function of the result of the comparison in step k), in order to direct some or all of the cold gases contained in the tunnel so as to restore the value of said temperature difference to said setpoint value T0exit-entry if necessary.                said data acquisition and processing unit uses a regulator of the PID type in order to carry out said feedback.        said extraction means on which the feedback is carried out comprise a single extraction line located inside the tunnel, substantially above the region where the products enter.        
The invention also relates to a cryogenic tunnel which incorporates such operating means as described above.